Prior art superconducting element joints are usually provided by inserting an insert element with each end connecting each piece of superconducting cable to thereby form a double joint.
Such superconducting element joints are e.g. known from US2005/0067184A1.
In the prior art several types of superconducting elements which are desired to joint is known. Such super conducting elements include superconducting busbars (in the following just called ‘busbars’), superconducting cables, layers of superconducting cables and other similar elements.
For long length cable systems the technology of cable joining is necessary because the individually superconducting elements cannot be produced in sufficient length without joining/splicing elements
The superconducting cables known today includes in particular 3 major types, namely cold-dielectric co-axial design, warm-dielectric single-phase cables, and tri-axial cable design, i.e. three concentric phases centered around a carrier, former or similar and surrounded by a concentric electrical screen e.g. as described in WO06/111170, U.S. Pat. No. 6,750,399 and/or in EP1053193
The tri-axial cable design has several advantages over other HTS cable designs. The advantages over a cold-dielectric co-axial design include:                Reduced use of superconducting material (e.g. by 34-50%) leading to reduced cost and reduced energy loss.        Reduced use of cryogenic envelope materials and cold surface (e.g. by 30-50%) leading to reduced cost and increased energy efficiency.        
Advantages of the tri-axial cable design compared to warm-dielectric single-phase cables include:                Essentially no external magnetic fields creating disturbances externally to the cable.        Improved relation between the electrical properties of inductance and capacitance leading to longer critical lengths, improved stability and reduced load-dependant voltage drops.        Reduced magnetic fields internally in the cable leading to lower energy losses and improved performance of the superconducting materials.        Reduced use of cryogenic envelope materials and cold surface (e.g. by a factor 30-50%) leading to reduced cost and increased energy efficiency.        Reduced number of cryogenic envelopes leading to fewer welding and fabrication steps, lower fabrication costs and increased reliability.        
Disadvantages of the tri-axial cable design compared to the two alternative designs may be the following:                More complex cable design and termination design than the co-axial cold dielectric and the warm-dielectric single phase leading to higher risk in manufacturing and in utilization.        
As indicated above prior art have dealt with plug-in type of conductor splice, which constitute an insert or transition piece, either in Cu, another low resistivity conductor or a composite insert made where the superconducting wires/tapes are embedded in a matrix. Although using a plug-in type insert reduces the complexity of the splice handling it introduces at least two conductor joints for each conductor and in the case of a Cu transition piece, it also introduces an extended ohmic transition. Another point that is not addressed in prior art is the undesired ac loss of the prior art conductor joint. Ohmic losses can be reduced by forced cooling. However, for a long cable the ohmic joint losses accumulate and it will have to be reduced or removed by cooling together with other losses. In this case the ohmic loss adds to the cryogenic losses as well as the magnetically generated ac losses of a superconducting ac cable. For a dc cable the contribution to the resulting loss is even more significant because no losses are generated in the superconductor.